1. Technical Field
The present disclosure generally relates to optical sensors, and more specifically, to interferometric optical sensors using a modulated emission of light for displacement measurement.
2. Description of the Related Art
Hearing aids, among a number of other electronic devices, provide specific applications in which miniature displacement measurement devices may be used. Tiny microphone arrays are currently housed in hearing aids to pick up slight acoustic pressures. Conventional microphones such as those used in hearing aids measure a change in capacitance between two conducting plates, one of which (the microphone diaphragm) moves as a function of the acoustic pressure applied. However, other types of miniature displacement measurement devices have been proposed that use optical detection such as anomalous diffraction or interferometry.
For example, exemplary methods and devices for anomalous diffraction are described in the journal article “Laterally Deformable Nanomechanical Zeroth-Order Gratings: Anomalous Diffraction Studied by Rigorous Coupled-Wave Analysis” by Carr D W, Sullivan J P, Friedmann T A, OPTICS LETTERS 28 (18): 1636-1638, September 2003, which is hereby incorporated by reference in its entirety. Optical interferometry, on the other hand, refers to the splitting and recombining of electromagnetic waves, in particular, light waves, to measure surface geometries, distance, etc. Micro-machinery is a growing technology field that often utilizes interferometers because they typically have high resolution and precision. In general, displacement measurements in the sub-nanometer range can be detected with today's interferometers.
U.S. Pat. No. 6,567,572 and U.S. patent application Ser. No. 10/704,932 to Degertekin, et al. disclose a number of displacement sensor embodiments generally comprising a reflector and a diffraction grating spaced a distance from the reflector. The devices can be used measure the displacement of the reflector from the diffraction grating. However, actual implementation of the displacement sensors in portable devices, such as hearing aids, has been hampered by the power use of these sensors. In particular, the power used to illuminate the light source can quickly discharge the batteries typically used in hearing aids.
Although semiconductor laser diodes are known to be more power efficient in comparison to, for example, a stabilized laser such as a Helium-Neon (HeNe) laser, semiconductor lasers have not been favored in high-sensitivity optical detection devices such as displacement sensors because of their thermal instability, low coherence length, and high laser intensity noise.
Additionally, the photo-detectors used for measuring the reflected and diffracted light from the grating and reflector can also introduce undesirable noise into the resulting sensor output.
Accordingly, systems and methods are needed for reducing the power consumption of high-sensitivity optical sensors. Additionally, systems and methods are needed for reducing the effect of thermal instability and/or noise associated with sensors using semiconductor laser diodes and detection electronics.